1. Field of the Invention
The present invention relates to a fisheye lens sufficiently securing an equivalent air distance between the last lens surface and the image plane, in spite of its compactness, suitable for a SLR camera and in particular for a SLR digital camera.
2. Related Background Art
Almost all fisheye lenses for a SLR camera have an image size corresponding to 35-mm films (For example, Japanese Patent Application Laid-Open No. 54-32319.) Even if these fisheye lenses are directly used in a SLR digital camera, the angle of view of 180 degrees cannot be secured because due to the image size the relation between the focal length and the angle of view of a SLR digital camera is different from that of a SLR film camera.
Generally, the size of the imaging plane of an imaging device for a SLR digital camera is a little smaller than the film frame size of 35-mm films. Accordingly, “the diagonal of the image size of 35-mm film frame” divided by “the diagonal of the image size of the imaging device” multiplied by the focal length of the lens becomes the focal length (converted focal length to 35-mm film format) of the lens used in a SLR digital camera.
Therefore, when a fisheye lens for a SLR film camera having the angle of view of 180 degrees is directly used in a SLR digital camera, no more than n effect of a super wide-angle lens can be obtained.
As described above, the angle of view of a SLR digital camera whose image size is a little smaller than that of a SLR film camera inevitably becomes narrow. Moreover, when you want to take a photograph of a wide area with use of a fisheye lens, the focal length has to be set even shorter. As a result, since the back focal length of a fisheye lens for a SLR digital camera is required to become more than 3 times as large as the focal length, an extremely strong divergent lens group has to be located to the object side of the lens system.
In a fisheye lens, a retrofocus type in which the principal point is located backward is used. However, since an extremely strong divergent lens group is located to the object side of the lens system, optical performance tends to be deteriorated due to curvature of field and astigmatism.
Moreover, in the retrofocus type, the diameter of the negative lens in the front lens group tends to become large, so that it is a problem that the fisheye lens becomes larger and heavier.